1. Field of the Invention
The present invention relates to novel compositions based on polyamides and rendered flame-retardant by means of red phosphorus, and to the use of such compositions for the production of shaped articles for the electrical and electronic industries.
This invention especially relates to novel polyamide compositions useful for the production of flame-retardant shaped articles which offer a good compromise of properties, in particular with regard to fire and flame resistance, resilience, resistance to arc tracking and surface appearance, as well as low corrosiveness.
2. Description of the Prior Art
It is known to this art that the addition of red phosphorus to compositions based on polyamides imparts a high degree of flame retardancy thereto.
U.S. Pat. No. 3,883,475 describes compositions based on flame-retardant polyamides that purportedly avoid the evolution of highly toxic phosphine, which forms via the disproportionation of red phosphorus under the influence of the trace amounts of water present in the polymer and of the high temperature required for its conversion, these compositions having incorporated therein a metal compound which advantageously is cupric oxide.
Improved such compositions, incorporating cadmium oxide instead of CuO, are described in French Patent No. 2,367,100; in addition to its effectiveness (at least equal to that of CuO) in preventing the evolution of phosphine, cadmium oxide provides shaped articles which have a particularly high resistance to arc tracking (as defined in French Standard NF C 26220), which is equal to or greater than 400 volts in the case of compositions containing no fillers and equal to or greater than 375 volts in the case of compositions filled with more than 25% by weight of glass fibers.
A replacement product for cadmium oxide (which has some toxicity), namely, a compound based on a lanthanide, is described in French Patents Nos. 2,553,783 and 2,621,594. With this adjuvant, the final compositions based on flame-retardant polyamides provide shaped articles which offer a compromise of properties, in particular with regard to flame retardancy, resilience and resistance to arc tracking, which is better than that associated with the incorporation of cadmium oxide.
However, these compositions flame-retarded by means of red phosphorus and optionally comprising other components to prevent the evolution of phosphine exhibit a certain corrosiveness with respect to metal parts which are in contact with the polymer or inserted, for example by molding, into an article shaped from these compositions. This phenomenon is observed, in particular, with molded parts used in the electrical or electronics field to support metal contacts, for example copper contacts.